libtime: Use Duration in Timespec arithmetic.

This changes the `Add` and `Sub` implementations for `Timespec` introduced in #16573 to use `Duration` as the time span type instead of `Timespec` itself, as [suggested](https://github.com/rust-lang/rust/pull/16573#issuecomment-52593408) by @sfackler.

This depends on #16626, because is uses `Duration::seconds(i64)`, whereas currently `Duration::seconds` takes an `i32`.